Organic light-emitting diodes (OLEDs) have been widely used in a small-size flat-panel display device such as mobile phone or flat-panel computer, and an integration degree and power consumption of a driving circuitry of the display device are highly demanded. A charge pump is a preferred step-up/step-down circuitry for a power source management module of the small-size display device due to such features as simple structure and high integration degree. However, usually a lithium battery is adopted by the small-size OLED display device, and its service voltage may decline along with an increase in its service time.